Radiofrequency magnetic field resonator and a method of designing the same

ABSTRACT

A radiofrequency (RF) resonator for magnetic resonance analysis, the RF resonator comprising: (a) at least two conductive elements, each having a first curvature along a direction perpendicular to a longitudinal axis, the at least two conductive elements being spaced along the longitudinal axis, so that when an RF current flows within the at least two conductive elements in a direction of the longitudinal axis, a substantially homogenous RF magnetic field, directed perpendicular to the longitudinal axis, is produced in a volume defined between the at least two conductive elements. The RF resonator further comprises (b) an electronic circuitry designed and configured for providing predetermined resonance characteristics of the RF resonator, for matching an impedance of the RF resonator to an impedance of an RF transmitter electrically communicating with the electronic circuitry, and for balancing the RF magnetic field to have a substantially symmetrical profile with respect to a transverse axis being perpendicular to the longitudinal axis.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to magnetic resonance analysis and, moreparticularly, to a radiofrequency (RF) resonator for generating asubstantially homogenous RF magnetic field for the purpose of magneticresonance analysis. The present invention further relates to a method ofdesigning the RF resonator, a magnetic resonance imaging (MRI) apparatusincorporating the RF resonator and a method of magnetic resonanceanalysis of an object.

MRI is a method to obtain an image representing the chemical andphysical microscopic properties of materials, by utilizing a quantummechanical phenomenon, known as Nuclear Magnetic Resonance (NMR), inwhich a system of spins, placed in a magnetic field resonantly absorbenergy, when applied with a certain frequency.

A nucleus can experience NMR only if its nuclear spin is not zero, i.e.,the nucleus has at least one unpaired nucleon. When placed in a magneticfield, a nucleus having a spin is allowed to be in a discrete set ofenergy levels, the number of which is determined by the spin, and theseparation of which is determined by the gyromagnetic ratio of thenucleus and by the magnetic field. Under the influence of a smallperturbation, manifested as an RF magnetic field, which rotates aboutthe direction of a primary static magnetic field, the nucleus has a timedependent probability to experience a transition from one energy levelto another. With a specific frequency of the rotating magnetic field,the transition probability may reach the value of unity. Hence atcertain times, a transition is forced on the nucleus, although therotating magnetic field may be of small magnitude relative to theprimary static magnetic field. For an ensemble of nuclei the transitionsare realized through a change in the overall magnetization.

Most MRI systems use a static magnetic field having a predeterminedgradient, so that a unique magnetic field is generated at each region ofthe analyzed object. By detecting the NMR signal, knowing the magneticfield gradient, the position of each region of the object can be imaged.Typical MRI systems include a main magnet generating a uniform staticmagnetic field, whereas gradients in predetermined directions areobtained by providing additional coils which generate the desiredgradients.

The rotating magnetic field in MRI systems is provided by an RFresonator, also known as RF coil, RF probe or RF antenna. The process ofimaging or analyzing an object (e.g., a patient or a sample) is asfollows. When pulse sequences are applied to the RF resonator, an RFradiation is emitted onto the object. According to the above principles,the RF radiation triggers NMR signals from the object from whichinformation is obtained and subsequently used to reconstruct imagesand/or to analyze the object. In most MRI systems, the RF resonator isused both for transmitting the RF radiation and for detecting theresulting NMR signals from the object. RF resonators are required togenerate a very uniform RF magnetic field, as any inhomogeneity in theRF magnetic field causes identical spins at different locations withinthe imaged object to respond differently to the RF radiation therebydistorting the image or negatively affecting the quality of theanalysis. It is recognized that RF resonators which generateinhomogeneous magnetic fields also have inhomogeneous sensitivity andthat RF resonators generating a weak magnetic field also detect weak NMRsignals.

In addition to the homogeneity requirement, the RF magnetic fieldgenerated by the RF resonator is required to have a resonance frequencywhich matches the resonance frequency of the nuclear spins in the imagedsample. A known phenomenon is that once a sample is inserted into the RFmagnetic field, the resonance frequency is shifted. Thus, RF resonatorsare typically equipped with appropriate circuitries which tune andrematch the resonance frequencies of the RF resonator and the sample. Toincrease signal-to-noise-ratio (SNR) and to optimize the efficiency ofthe system, it is also desirable that the size of the RF resonator willbe comparable to the size of the sample.

Many RF resonators are presently known, and can be categorized into twogroups, commonly referred to as the group of surface resonators and thegroup of volume resonators.

FIG. 1 illustrates a surface resonator, known as the single-loop coil[M. R. Bendall, “Surface Coil Technology”, Magnetic Resonance Imaging,ed. by C. L. Partain et al., Philadelphia, 1988]. The single-loop coilis a planar current-loop, which is sensitive to RF fields, designatedherein by B₁, in the direction perpendicular to the loop surface. Thesingle-loop coil is characterized by a high SNR, however, its effectivehomogenous RF field is only near the surface, or the plane, of thecurrent-loop. Thus, the single-loop coil is highly inhomogeneous.

FIG. 2 illustrates another surface RF resonator, known as the phasedarray coil [Sodickson D K and Manning W J., “Simultaneous acquisition ofspatial harmonics (SMASH): ultra-fast imaging with RF coil arrays”,Proc. Fifth Scientific Meeting of the International Society for MagneticResonance in Medicine, 1817 (1997); Klass P. Pressmann, et al., “SENSE:Sensitivity Encoding for fast MRI”, Magnetic Resonance in Medicine,42:952, 1999]. The phased array coil is an array of a number ofsingle-loop coils, where the phases of the single-loop coils aredesigned so that the matrix representing their signals is can bediagonalized. The volume of interest of the phased array coil is thecombined near-surface of each single unit of the array.

As opposed to the surface resonators, where the effective imaged regionis a surface (i.e., two-dimensional space), in the group of volumeresonators the effective imaged region is a volume (three-dimensionalspace). FIG. 3 illustrates a volume RF resonator known as a saddle coil[D. W. Alderman et al., J. Magn. Reson. 36, 447, 1979]. The saddle coilis made of a combination of two current-loops, wrapped around a lateralsurface area of a cylinder, so that the magnetic fields of the twocurrent-loops combine. In some saddle coils each loop is made amulti-loop structure in a spiral manner.

FIG. 4 illustrates another volume RF resonator known as the multi-turnsolenoid [D. I. Hoult and P. C. Lauterbour, J., “The Sensitivity ofZeugmatographic Experiment Involving Human Samples” Magn. Reson, 34:425,1979 ]. The multi-turn solenoid is a structure which generates a veryhomogenous magnetic field within a cylindrical volume. The multi-turnsolenoid is rarely used in MRI because the magnetic field is parallel tothe cylinder axis and is therefore orthogonal to the symmetry ofclinical systems where the RF field is to be orthogonal to thelongitudinally oriented main static magnetic field.

FIG. 5 illustrate an additional volume RF resonator known as thesingle-turn solenoid [J. P. Hornak, et al., “Elementary Single TurnSolenoids Used in the Transmitter and Receiver in Magnetic ResonanceImaging”, Magn. Reson. Imag., 5:233-237, (1987)]. The single-turnsolenoid formed by a broad sheet instead of a multi-turn structure.Conceptually, it is very similar to the conventional solenoid, thus, itsuffers from similar limitations as the multi-turn solenoid and it istherefore used only for imaging of particular samples, such as thebreast and forearm. One type of single-turn solenoid is the perforatedsingle-turn solenoid which is a single-turn solenoid with one or moreperforations in its cylinder. The perforated single-turn solenoid issuitable for imaging of head, knee, elbow, wrist and shoulder.

FIG. 6 illustrates the most common volume RF resonator for clinicalimaging, known as the birdcage resonator or the birdcage coil [C. E.Hayse et al., “An Efficient Highly Homogeneous Radiofrequency Coil forWhole-Body NMR Imaging at 1.5T”, J. Magn. Reson., 63:622-628, 1985; J.Tropp, “The theory of the bird-cage resonator”, J. Magn. Reson. 1989;82, 51-62; T. A. Riauka el al., “A numerical approach to non-circularbirdcage RF coil optimization: Verification with a fourth-order coil”,Magnetic Resonance in Medicine, 41:1180-1188, 1999; Jianming J.“Electromagnetic analysis and design in magnetic resonance imaging”, CRCPress, New York, 1999]. The birdcage coil is a very homogenous resonatorwhich is formed from a number of equally spaced conductors on acylindrical surface. It is common to refer to conductors which arelongitudinally oriented with respect to the symmetry axis of thecylinder as “legs” or “rungs”, and to conductors which are transversallyoriented as “end-rings”. Capacitors are located on the legs, on theend-rings or both on the legs and the end-rings.

Currents flowing in the legs and end-rings obey an eigenvalue equationwhich is characterized by a set of eigenvectors, also known ascurrent-modes. Each current-mode corresponds to a set of currentsflowing in the legs, and is associated to an eigenvalue which is relatedto one possible solution of resonant frequency. Specifically, a birdcagecoil with N legs has N resonant frequencies and N current-modes. For alinear birdcage, one special current-mode, has a sinusoidal currentdistribution among the legs of each side along the circumference. Inthis mode, the magnetic field inside the resonator is very homogenous.In is known that the homogeneity level of the field is proportional tothe number of legs, where an infinite number of legs incorporate adesired mode corresponding to an RF field having a perfect homogeneity.

However, as the number of current-modes increases, so does thecomplexity of the birdcage coil design. The undesired current-modes ofthe birdcage coil, except for intrinsically orthogonal cosine mode,reduce the field homogeneity and/or the transmission and detectionpower. Thus, when designing a birdcage coil, one needs to eliminate allcurrent-modes other then the desired mode to ensure a homogenous field.This is typically done by designing a birdcage coil where the desiredmode resonant frequency is significantly spaced apart from all the otherfrequencies. An additional factor which is to be considered whenselecting the number of birdcage coil legs is the physiological effectof a large number of legs on the patient which may becomeclaustrophobic.

An inherent limitation of the birdcage coil is that in the designprocess, both the magnetic field characteristic and the resonancecharacteristic of the birdcage coil are to be simultaneously calculated,as these two characteristics are entangled. Any change in the number oflegs and separation therebetween and the location of the capacitors andthe type thereof alters both the RF field lines and the resonantfrequency of the coil.

Moreover, the simulations preceding the manufacturing of the birdcagecoil exhibits a very homogenous RF magnetic field. In practice, however,once a sample is inserted into the coil, the resonance frequency and thetuning impedance of the coil are shifted. Although this effect can beapproximated during simulation, the validity of such approximation isvery limited due to the various sizes, structures and orientations thata biological organ may exhibit when placed in the birdcage coil,contrary to the spherical or cylindrical uniform sample that istypically used for simulations. Thus, the resonance frequency and thetuning impedance must be retuned by an arrangement of tunablecapacitors, which has to be electrically connected to the birdcage coil.Theoretically, N tunable capacitors can correct some of the sampleeffects, but for practical reasons only a few capacitors are used. Theadditional capacitors break the symmetry of the birdcage coil, result inloosing field homogeneity and introduce non-zero contributions from oneor more undesired current-modes. In realistic birdcage coils, theinhomogeneity of the RF field may approach 15-20%.

Additional prior art of relevance is a volume RF resonator known as theLitz coil, disclosed in U.S. Pat. No. 6,060,882 and illustrated in FIG.7. The coil is based on Litz foil conductor, which includes multiple andparallel Litz wires with interwoven sub-routes from a first node to asecond node and insulated crossovers forming well-defined fluxsub-windows. The structure of the coil results in multiple currentroutes each contribute to the RF magnetic field, B₁, leaving a centralflux window centered on the B₁ axis. An identical semi-coil, formed onthe opposite side of the sample around completes the coil. The twosemi-coils are electrically connected in parallel. This coil solves someof the problems associated with the birdcage coil, however its designand manufacturing is still rather complicated. Specifically, the Litzcoil efficiency depends on the number of braids and on their thickness.Hence, for a sufficiently efficient Litz coil the braiding is verycomplex and ultra thin.

Also of prior art of relevance is the so-called “Slotted Tube Resonator”[H. J. Schneider and P. Dullenkopf, “Slotted Tube Resonator: A new NMRprobe head at high observing frequencies”, Rev. Sci. Instrum., 48:68-73,1977]. This resonator is composed of a conducting tube which is cutlengthwise, thereby forming a strip line which consists of two archedconductors. The slotted tube resonator is coupled to an electroniccircuitry, which includes two independently operating capacitors, aseries capacitor for matching the resonant characteristics of theresonator and a shunting capacitor for tuning the impedance of theresonator.

An improved type of the slotted tube resonator [D. H. Hong et al.,“Whole Body Slotted Tube Resonator for Proton NMR Imaging at 2.0 Tesla”,Magn. Res. Imag., 5:239-243, 1987;], incorporates a coupling sheet,positioned externally to the tube arches, for linking the resonator withthe transmitter and/or the receiver of the imaging system. In addition,the improved slotted tube resonator includes additional capacitorsforming a capacity coupling between the two tube arches.

One application of the slotted tube resonator, designed specifically foranalyzing hearts of rabbits of different sizes, is described in G. J.Kost, “A Cylindrical-Window NMR Probe with Extended Tuning Range Studiesof the Developing Heart”, J. Magn. Res. 82:238-252, 1989. This coil iscylindrical and it is formed with a window in the cylinder wall. Unlikethe multi- and single-turn solenoids, the produced magnetic field of thecylindrical windowed coil is directed perpendicularly to the symmetryaxis of the cylinder. This coil, however, is suitable only fornon-imaging applications of small sized samples.

Other slotted tube resonators are described in S. Bobroff and M. J.McCarthy, “Variations on the Slotted-Tube Resonator: Rectangular andElliptical Coils”, Magn. Res. Imag., 17:783-789, 1999; M. K. Murphy etal., “A Comparison of Three Radiofrequency Coils for NMR Studies ofConductive Samples”, Magn. Res. in Med., 12:382-389, 1989; T.Sphicopoulos and F. Gardiol, “Slotted Tube Cavity: a Compact ResonatorWith Empty Core”, IEE Proceedings, 134:405-410; A. Darrasse et al., “Theslotted cylinder: an efficient probe for NMR imaging, Magnetic Resonancein Medicine, 2(l):20-8, 1985; H. J. Schneider and P. Dullenkopf,“Crossed Slotted Tube Resonator: A new Double resonance NMR probehead”,Rev. Sci. Instrum., 48:832-834, 1977; D. W. Alderman and D. M. Grant,“An Efficient Decoupler Coil Design which Reduces Heating in ConductiveSamples in Superconducting Spectrometers”, J. Magn. Res., 36:447-451,1979; S. Crozier et al., “In Vivo Localized ¹H NMR Spectroscopy at 11.7Tesla”, J. Magn. Res., 94:123-132, 1991; C. Ranasinghage et al.,“Resonator Coils for magnetic resonance imaging at 6 MHz”, Med. Phys.15:235-240, 1988; and T. A. Gross et al., “Radiofrequency Resonators forHigh-Field Imaging and Double-Resonance Spectroscopy”, J. Magn. Res.,62:87-98, 1985.

It will be appreciated that, all of the prior art slotted tuberesonators suffer from two crucial limitations: (i) the slotted tuberesonator fails to provide a mechanism for balancing the RF fieldhomogeneity once it has deviated from its original design due tosample-field interactions; and (ii) the geometrical configuration of theslotted tube resonator is such that the generated RF magnetic field hasa linear polarization. An intrinsic limitation of the linearpolarization is the power losses, which are explained by wastedcomponents in the mathematical expansion of the linear polarization. Ina theoretical study of current distributions and field uniformity insaddle coils [J. W. Carlson, “Currents and Fields of Thin Conductors inRF Saddle Coils”, Magn. Res. in Med., 3:778-790, (1986)] an optimalgeometry for circular polarized coil has been calculated.

In this respect, one coil for generating a circular polarized magneticfield is of the birdcage type described above, also known as thequadrature birdcage coil, in which intrinsically orthogonal sine andcosine modes are incorporated. A particularly interesting quadraturebirdcage coil is described in an article by H. Barfuss et al., entitled“In Vivo Magnetic Resonance Imaging and Spectroscopy of Humans with a 4TWhole-body Magnet”, published in NMR in Biomedicine, 3:31-45. Thisbirdcage coil consists of four longitudinal and two transverse copperfoils, interconnected via a system of capacitors. The coil is furtherequipped with additional rotary differential capacitors for providingcontinuous distribution between the tuning and matching capacitors,thereby allowing variation of the quality factor of the coil. However,this birdcage coil only partially addresses to problems associated witheffects of interaction between the magnetic field and the imaged object.Specifically, this birdcage coil, although successfully providing asophisticated circuitry for tuning the resonance characteristics andmatching the impedance, fails to provide a satisfactory mechanism forcorrecting inhomogeneity in the magnetic field once a sample is placedtherein.

There is thus a widely recognized need for, and it would be highlyadvantageous to have, an RF resonator and a method of designing thesame, devoid of the above limitations.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided aradiofrequency (RF) resonator for magnetic resonance analysis, the RFresonator comprising: (a) at least two conductive elements, each havinga first curvature along a direction perpendicular to a longitudinalaxis, the at least two conductive elements being spaced along thelongitudinal axis, so that when an RF current flows within the at leasttwo conductive elements in a direction of the longitudinal axis, asubstantially homogenous RF magnetic field, directed perpendicular tothe longitudinal axis, is produced in a volume defined between the atleast two conductive elements; and (b) an electronic circuitry designedand configured for providing predetermined resonance characteristics ofthe RF resonator, for matching an impedance of the RF resonator to animpedance of an RF transmitter electrically communicating with theelectronic circuitry, and for balancing the RF magnetic field to have asubstantially symmetrical profile with respect to a transverse axisbeing perpendicular to the longitudinal axis.

According to another aspect of the present invention there is provided amethod of designing a radiofrequency (RF) resonator for magneticresonance analysis, the method comprising: (a) selecting at least twosurfaces to engage at least two conductive elements, the at least twosurfaces having a first curvature along a direction perpendicular to alongitudinal axis, thereby defining a geometry between the at least twosurfaces; (b) using the geometry for calculating a magnetic field withinthe ; (c) iteratively repeating the steps (a) and (b) so as to provideoptimized geometry corresponding to a substantially homogenous magneticfield; and (d) using the optimized geometry and the substantiallyhomogenous magnetic field for designing an electronic circuitry forproviding predetermined resonance characteristics of the RF resonator,for matching an impedance of the RF resonator to an impedance of an RFtransmitter electrically communicating with the electronic circuitry,and for balancing the RF magnetic field to have a substantiallysymmetrical profile with respect to a transverse axis beingperpendicular to the longitudinal axis.

According further features in the preferred embodiments described below,the matching is by varying mutual capacitance.

According to still further features in the described preferredembodiments the matching is by varying mutual inductance.

According to still further features in the described preferredembodiments the calculating the magnetic field within the geometry is bysolving Maxwell's equations.

According to still further features in the described preferredembodiments the calculating the magnetic field within the geometry is byfinite element method.

According to still further features in the described preferredembodiments the calculating the magnetic field within the geometry is bymoments analysis method.

According to still further features in the described preferredembodiments the method further comprising designing an RF shield forminimizing electromagnetic interactions between the RF resonator and atleast one gradient coil and/or between the RF resonator and a device forproviding a static magnetic field.

According to still further features in the described preferredembodiments the designing an RF shield is by the method of images.

According to still further features in the described preferredembodiments the method further comprising designing at least one end-capto be positioned adjacent to at least one end of the RF resonator forminimizing magnetic field inhomogeneities along the longitudinal axis.

According to still further features in the described preferredembodiments the method further comprising designing at least oneadditional conductive element, so as to further minimize inhomogeneityof the magnetic field.

According to still further features in the described preferredembodiments a phase of an RF current flowing through the at least oneadditional conductive element equals a phase of currents flowing throughthe at least two conductive elements.

According to still further features in the described preferredembodiments an RF current flowing through the at least one additionalconductive element depends on currents flowing through the at least twoconductive elements, through a predetermined function.

According to still further features in the described preferredembodiments an RF current flowing through the at least one additionalconductive element is a predetermined fraction of currents flowingthrough the at least two conductive elements.

According to yet another aspect of the present invention there isprovided an apparatus for magnetic resonance analysis, the apparatuscomprising: (a) a device for providing a static magnetic field; (b) aprocessing unit; and (c) an RF resonator coupled to an RF transmitter,the RF resonator comprising: at least two conductive elements, eachhaving a first curvature along a direction perpendicular to alongitudinal axis, the at least two conductive elements being spacedalong the longitudinal axis, so that when an RF current flows within theat least two conductive elements in a direction of the longitudinalaxis, a substantially homogenous RF magnetic field, directedperpendicular to the longitudinal axis, is produced in a volume definedbetween the at least two conductive elements; and an electroniccircuitry designed and configured for providing predetermined resonancecharacteristics of the RF resonator, for matching an impedance of the RFresonator to an impedance of the RF transmitter, and for balancing theRF magnetic field to have a substantially symmetrical profile withrespect to a transverse axis being perpendicular to the longitudinalaxis.

According to still another aspect of the present invention there isprovided a method for Magnetic Resonance analysis of an object, themethod comprising: applying a static magnetic field on the subject in adirection of a longitudinal axis; applying a substantially homogenous RFmagnetic field on the subject, in a direction perpendicular to thelongitudinal axis; and acquiring nuclear magnetic resonance parametersfrom the object, thereby analyzing the object; wherein the applying thesubstantially homogenous RF magnetic field is by a RF resonator coupledto an RF transmitter, the RF resonator comprising: at least twoconductive elements, each having a first curvature along a directionperpendicular to the longitudinal axis, the at least two conductiveelements being spaced along the longitudinal axis, so that when an RFcurrent flows within the at least two conductive elements in a directionof the longitudinal axis, the substantially homogenous RF magneticfield, is produced in a volume defined between the at least twoconductive elements; and an electronic circuitry designed and configuredfor providing predetermined resonance characteristics of the RFresonator, for matching an impedance of the RF resonator to an impedanceof an RF transmitter electrically communicating with the electroniccircuitry, and for balancing the RF magnetic field to have asubstantially symmetrical profile with respect to a transverse axisbeing perpendicular to the longitudinal axis.

According to further features in the preferred embodiments describedbelow, the method further comprising balancing the RF magnetic fieldusing a balancing adjuster electrically communicating with theelectronic circuitry.

According to still further features in the described preferredembodiments the RF resonator further comprising at least one additionalconductive element positioned so as to further minimize inhomogeneity ofthe magnetic field.

According to still further features in the described preferredembodiments a phase of an RF current flowing through the at least oneadditional conductive element equals a phase of the RF current flowingthrough the at least two conductive elements.

According to still further features in the described preferredembodiments the RF resonator is characterized by two phases of RFcurrents, the two phases differ by 180 degrees.

According to still further features in the described preferredembodiments the method further comprising applying at least one gradientpulse on the object.

According to still further features in the described preferredembodiments a separation between the at least two conductive elements isselected so as to surround the object.

According to still further features in the described preferredembodiments the method further comprising preserving the at least twoconductive elements at a sufficiently low temperature.

According to still further features in the described preferredembodiments the applying the substantially homogenous RF magnetic fieldis by at least one additional RF resonator.

According to still further features in the described preferredembodiments the at least one additional RF resonator is arranged withthe RF resonator to form an RF resonator array.

According to still further features in the described preferredembodiments the method further comprising electrically decoupling the RFresonator from the at least one additional RF resonator.

According to still further features in the described preferredembodiments the method further comprising electromagnetically decouplingthe RF resonator from the at least one additional RF resonator.

According to still further features in the described preferredembodiments each of the at least two conductive elements has apredetermined capacitance distribution for minimizing effects of theobject on the magnetic field and for minimizing corona discharge fromthe at least two conductive elements.

According to an additional aspect of the present invention there isprovided an RF resonator for magnetic resonance analysis, the RFresonator comprising: (a) at least two conductive elements, each havinga first curvature along a direction perpendicular to a longitudinalaxis, the at least two conductive elements being spaced along thelongitudinal axis, so that when an RF current flows within the at leasttwo conductive elements in a direction of the longitudinal axis, asubstantially homogenous RF magnetic field, directed perpendicular tothe longitudinal axis, is produced in a volume defined between the atleast two conductive elements; and (b) at least one additionalconductive element, electrically communicating with the at least twoconductive elements in a manner such that a phase of an RF currentflowing through the at least one additional conductive element equals aphase of at least one of the RF currents flowing through the at leasttwo conductive elements.

According to further features in the preferred embodiments describedbelow, the RF resonator further comprising an electronic circuitrydesigned and configured for providing predetermined resonancecharacteristics of the RF resonator, for matching an impedance of the RFresonator to an impedance of an RF transmitter electricallycommunicating with the electronic circuitry, and for balancing the RFmagnetic field to have a substantially symmetrical profile with respectto a transverse axis being perpendicular to the longitudinal axis.

According to still further features in the described preferredembodiments the RF resonator further comprising means for preserving theat least two conductive elements at a sufficiently low temperature.

According to yet an additional aspect of the present invention there isprovided an apparatus for magnetic resonance analysis, the apparatuscomprising: (a) a device for providing a static magnetic field; (b) aprocessing unit; and (c) an RF resonator coupled to an RF transmitter,the RF resonator comprising: at least two conductive elements, eachhaving a first curvature along a direction perpendicular to alongitudinal axis, the at least two conductive elements being spacedalong the longitudinal axis, so that when an RF current flows within theat least two conductive elements in a direction of the longitudinalaxis, a substantially homogenous RF magnetic field, directedperpendicular to the longitudinal axis, is produced in a volume definedbetween the at least two conductive elements; and at least oneadditional conductive element, electrically communicating with the atleast two conductive elements in a manner such that a phase of an RFcurrent flowing through the at least one additional conductive elementequals a phase of at least one of the RF currents flowing through the atleast two conductive elements.

According to further features in the preferred embodiments describedbelow, the RF resonator further comprising an electronic circuitrydesigned and configured for providing predetermined resonancecharacteristics of the RF resonator, for matching an impedance of the RFresonator to an impedance of the RF transmitter, and for balancing theRF magnetic field to have a substantially symmetrical profile withrespect to a transverse axis being perpendicular to the longitudinalaxis.

According to still further features in the described preferredembodiments the RF resonator further comprising a balancing adjusterelectrically communicating with the electronic circuitry, the balancingadjuster is constructed and designed for controlling the electroniccircuitry while the RF resonator is in medical use.

According to still further features in the described preferredembodiments an RF current flowing through the at least one additionalconductive element depends on the RF currents flowing through the atleast two conductive elements, through a predetermined function.

According to still further features in the described preferredembodiments the predetermined function is selected from the groupconsisting of a linear function, a polynomial function, an exponentialfunction, a rational function, a power function and any combinationthereof.

According to still further features in the described preferredembodiments an RF current flowing through the at least one additionalconductive element is a predetermined fraction of the RF currentsflowing through the at least two conductive elements.

According to still further features in the described preferredembodiments the predetermined fraction is one half.

According to still further features in the described preferredembodiments the device for providing the static magnetic field comprisesat least one shim coil.

According to still further features in the described preferredembodiments the apparatus further comprising at least one gradient coil.

According to still further features in the described preferredembodiments the apparatus further comprising an RF shield constructedand designed for minimizing electromagnetic interactions between the atleast one gradient coil and the device for providing a static magneticfield.

According to still further features in the described preferredembodiments the apparatus further comprising at least one end-cappositioned adjacent to at least one end of the RF resonator, the atleast one end-cap constructed and designed for minimizing magnetic fieldinhomogeneities along the longitudinal axis.

According to still further features in the described preferredembodiments the RF resonator is coupled to the RF transmitter via atransmission line.

According to still further features in the described preferredembodiments the RF resonator is coupled to the RF transmitter via an RFantenna.

According to still further features in the described preferredembodiments the electronic circuitry comprises means for varying mutualcapacitance.

According to still further features in the described preferredembodiments the electronic circuitry comprises means for varying mutualinductance.

According to still further features in the described preferredembodiments the mutual inductance is defined between the RF resonatorand the RF transmitter.

According to still further features in the described preferredembodiments the mutual inductance is defined between the electroniccircuitry and the RF transmitter.

According to still further features in the described preferredembodiments the electronic circuitry comprises an arrangement ofcapacitors, inductors, tunable capacitors and tunable inductors.

According to still further features in the described preferredembodiments the capacitors and the tunable capacitors are high powercapacitors.

According to still further features in the described preferredembodiments the high power capacitors are vacuum capacitors.

According to still further features in the described preferredembodiments a longitudinal dimension of the at least two conductiveelements is selected so as to minimize magnetic field inhomogeneitiesalong the longitudinal axis.

According to still further features in the described preferredembodiments a separation between the at least two conductive elements isselected so as to surround an object to be imaged.

According to still further features in the described preferredembodiments the object is a mammal.

According to still further features in the described preferredembodiments the object is an organ of a mammal.

According to still further features in the described preferredembodiments the object is a tissue.

According to still further features in the described preferredembodiments the object is a swollen elastomer.

According to still further features in the described preferredembodiments the object is a food material.

According to still further features in the described preferredembodiments the object is liquid.

According to still further features in the described preferredembodiments the object is at least one type of molecules present in thesolvent.

According to still further features in the described preferredembodiments the at least one type of molecules present in the solvent isselected from the group consisting of molecule dissolved in the solvent,a molecule dispersed in the solvent and a molecule emulsed in thesolvent.

According to still further features in the described preferredembodiments the first curvature is selected from the group consisting ofa curvature of cylinder, a curvature of an ellipsoid, a curvature of ahyperboloid, a curvature of a paraboloid and a curvature of an irregularsurface.

According to still further features in the described preferredembodiments at least one of the at least two conductive elements furtherhas a second curvature along a direction parallel to the longitudinalaxis.

According to still further features in the described preferredembodiments the first curvature and the second curvature are eachindependently constant curvatures.

According to still further features in the described preferredembodiments the first curvature and the second curvature are eachindependently variable curvatures.

According to still further features in the described preferredembodiments the second curvature is selected from the group consistingof a curvature of cylinder, a curvature of an ellipsoid, a curvature ofa hyperboloid, a curvature of a paraboloid and a curvature of anirregular surface.

According to still further features in the described preferredembodiments a number of the at least two conductive elements is selectedso that the substantially homogenous RF magnetic field is linearlypolarized.

According to still further features in the described preferredembodiments a number of the at least two conductive elements is selectedso that the substantially homogenous RF magnetic field is substantiallycircularly polarized.

According to still further features in the described preferredembodiments the at least two conductive elements are two conductiveelements.

According to still further features in the described preferredembodiments the at least two conductive elements are four conductiveelements.

According to still further features in the described preferredembodiments a first pair of the four conductive elements is magneticallydecoupled from a second pair of the four conductive elements.

According to still further features in the described preferredembodiments a first pair of the four conductive elements is electricallydecoupled from a second pair of the four conductive elements.

According to still further features in the described preferredembodiments a first pair of the four conductive elements iselectromagnetically decoupled from a second pair of the four conductiveelements.

According to still further features in the described preferredembodiments a first pair and a second pair of the four conductiveelements are positioned so that a transverse axis of the first pair issubstantially perpendicular to a transverse axis of the second pair.

According to still further features in the described preferredembodiments the at least two conductive elements are made of asuperconducting material.

According to still further features in the described preferredembodiments the apparatus further comprising means for preserving the atleast two conductive elements at a sufficiently low temperature.

According to still further features in the described preferredembodiments the apparatus further comprising at least one additional RFresonator arranged with the RF resonator to form an RF resonator array.

According to still further features in the described preferredembodiments the apparatus further comprising decoupling means fordecoupling the RF resonator from the at least one additional RFresonator.

According to still further features in the described preferredembodiments the apparatus further comprising decoupling means fordecoupling the RF resonator from the at least one gradient coil.

According to still further features in the described preferredembodiments the decoupling means comprise DC block capacitors.

According to still further features in the described preferredembodiments the array is a phased array.

According to still further features in the described preferredembodiments the RF resonator is a multi frequency RF resonator.

According to still further features in the described preferredembodiments each of the at least two conductive elements has apredetermined capacitance distribution for minimizing effects of anobject to be imaged on the magnetic field and for minimizing coronadischarge from the at least two conductive elements.

According to still further features in the described preferredembodiments the at least two conductive elements are designed andconstructed to minimize eddy currents generated therein.

According to still further features in the described preferredembodiments the at least two conductive elements are characterized by anRF shield structure, for substantially blocking RF radiation whiletransmitting low frequency radiation.

The present invention successfully addresses the shortcomings of thepresently known configurations by providing an RF resonator and a methodof designing an RF resonator, which enjoy properties far exceeding theprior art.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although methods and materialssimilar or equivalent to those described herein can be used in thepractice or testing of the present invention, suitable methods andmaterials are described below. In case of conflict, the patentspecification, including definitions, will control. In addition, thematerials, methods, and examples are illustrative only and not intendedto be limiting.

Implementation of the method and system of the present inventioninvolves performing or completing selected tasks or steps manually,automatically, or a combination thereof. Moreover, according to actualinstrumentation and equipment of preferred embodiments of the method andsystem of the present invention, several selected steps could beimplemented by hardware or by software on any operating system of anyfirmware or a combination thereof. For example, as hardware, selectedsteps of the invention could be implemented as a chip or a circuit. Assoftware, selected steps of the invention could be implemented as aplurality of software instructions being executed by a computer usingany suitable operating system. In any case, selected steps of the methodand system of the invention could be described as being performed by adata processor, such as a computing platform for executing a pluralityof instructions.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings. With specific reference now tothe drawings in detail, it is stressed that the particulars shown are byway of example and for purposes of illustrative discussion of thepreferred embodiments of the present invention only, and are presentedin the cause of providing what is believed to be the most useful andreadily understood description of the principles and conceptual aspectsof the invention. In this regard, no attempt is made to show structuraldetails of the invention in more detail than is necessary for afundamental understanding of the invention, the description taken withthe drawings making apparent to those skilled in the art how the severalforms of the invention may be embodied in practice.

In the drawings:

FIG. 1 is a schematic illustration of a prior art single-loop coil;

FIG. 2 is a schematic illustration of a prior art phased array coil;

FIG. 3 is a schematic illustration of a prior art saddle coil;

FIG. 4 is a schematic illustration of a prior art multi-turn solenoid;

FIG. 5 is a schematic illustration of a prior art single-turn solenoid;

FIG. 6 is a schematic illustration of a prior art birdcage coil;

FIG. 7 is a schematic illustration of a prior art Litz coil;

FIGS. 8 a-c are schematic illustrations of a radiofrequency resonatorfor magnetic resonance analysis, according to the present invention;

FIG. 9 is a flowchart of a method of designing a radiofrequencyresonator for magnetic resonance analysis, according to the presentinvention;

FIG. 10 is a block diagram illustrating an MRI apparatus, according tothe present invention;

FIG. 11 is a flowchart of a method for magnetic resonance analysis of anobject, according to the present invention;

FIGS. 12 a-f show the calculated equipotential lines of the field, forvarious profiles of the conductive elements of a prototype linear RFresonator, according to the present invention;

FIG. 13 is a schematic illustration of a circuit for AC simulations,according to the present invention;

FIG. 14 shows the simulated potential and potential difference betweenvarious points in the circuit of FIG. 13, according to the presentinvention;

FIG. 15 is a schematic illustration of the experimental setup of theprototype linear RF resonator, according to the present invention;

FIGS. 16 a-b show profile and statistical characteristics of the RFmagnetic field trough an axial slice of a phantom, imaged by theprototype linear RF resonator, according to the present invention;

FIGS. 17 a-b show profile and statistical characteristics of the RFmagnetic field trough a coronal slice of a phantom, imaged by theprototype linear RF resonator, according to the present invention;

FIGS. 18 a-b show profile and statistical characteristics of the RFmagnetic field trough a sagittal slice of a phantom, imaged by theprototype linear RF resonator, according to the present invention;

FIGS. 19 a-d show axial spin echo slices in a rat head, imaged by theprototype linear RF resonator according to the present invention;

FIG. 20 is a schematic illustration of a top view of a quadrature RFresonator, according to the present invention;

FIG. 21 is a schematic illustration of electrical currents induced ontothe surfaces of the conductive elements of the quadrature RF resonator,according to the present invention;

FIG. 22 is a schematic illustration of the voltage configuration of thequadrature RF resonator, according to the present invention;

FIG. 23 a shows calculated electric potential of the quadrature RFresonator, according to the present invention; and

FIG. 23 b shows calculated electric potential and electric field of thequadrature RF resonator, according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is of an RF magnetic field resonator which can beused for magnetic resonance analysis, such as MRI. Specifically, thepresent invention is of a simple-designed RF resonator which can be usedto provide a substantially homogeneous RF magnetic field in an MRIapparatus. The present invention is further of a method of designing theRF resonator, an MRI apparatus incorporating the RF resonator and amethod of magnetic resonance analysis of an object using the RFresonator.

The principles and operation of an RF resonator according to the presentinvention may be better understood with reference to the drawings andaccompanying descriptions.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangement of the components setforth in the following description or illustrated in the drawings. Theinvention is capable of other embodiments or of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting.

According to one aspect of the present invention there is provided an RFresonator for magnetic resonance analysis, generally referred to hereinas RF resonator 10.

Referring now to the drawings, FIGS. 8 a-c illustrate RF resonator 10,which comprises at least two conductive elements 12, spaced along alongitudinal axis 14, where each conductive element has a firstcurvature along a direction perpendicular to longitudinal axis 14.According to a preferred embodiment of the present invention any number(larger than two) of conductive elements may be used. Hence, an exampleof two conductive elements configuration is shown in FIG. 8 a, and anexample of four conductive elements configuration is shown in FIG. 8 b.Anti-parallel RF currents 16, flowing within conductive elements 12 in adirection of longitudinal axis 14, generate an RF magnetic field 18 in avolume 20, defined between conductive elements 12.

Magnetic field 18 is directed perpendicular to longitudinal axis 14, andit is substantially homogenous. A preferred characterization of thehomogeneity of magnetic field 18 is a magnetic field variation which isless than about 20% within about 80% of volume 20. More preferably, thehomogeneity of magnetic field 18 is characterized by a magnetic fieldvariation which is less than about 5% within about 50% of volume 20.

As used herein the term “about” refers to ±10%.

RF resonator 10 further comprises an electronic circuitry 22 which, asfurther detailed hereinunder, is primarily designed and configured forthree purposes, which are considered as three degrees-of-freedom of RFresonator 10: (i) providing predetermined resonance characteristics ofRF resonator 10; (ii) matching an impedance of RF resonator 10 to animpedance of an RF transmitter 24 electrically communicating withelectronic circuitry; and (iii) balancing the RF magnetic field to havea substantially symmetrical profile with respect to a transverse axis(i.e., perpendicular to longitudinal axis 14), hence correcting sampleeffects on circuitry 22.

Before providing a further detailed description of RF resonator 10, asdelineated hereinabove and in accordance with the present invention,attention will be given to the advantages and potential applicationsoffered thereby.

Hence, the construction and the design of RF resonator 10 allows foroptimizing the RF magnetic field generated therein so that the amount ofRF field inhomogeneity is substantially minimize, even when RF resonator10 is loaded with an object. More specifically, RF resonator 10 enjoysproperties exceeding all prior art coils, such as surface coils,birdcage coils and slotted tube coils (for further description of theseprior art coils, see the drawings and the accompanied description in theBackground section hereinabove).

With respect to prior art surface coils, whereas the transverse field ofsuch coils is very inhomogeneous except for a narrow surface, themagnetic field generated by RF resonator 10 is substantially homogenousthroughout the volume defined between conductive elements 12, thus, theentire object of interest can be imaged to a large extent of quality.

With respect to prior art birdcage coils, although, theoretically, thelarge number of difference-phased current-loops in these coils(typically of the order of ten) allow for generating a homogenous RFfield, birdcage coils are associated with an inherent limitation, inwhich the number of tunable capacitors which are needed to match andtune the birdcage coil is impractical. Hence, as stated, some capacitorsare fixed to a predetermined value of capacity, leaving only a smallportion of the available degrees-of-freedom to be corrected. In isrecognized that, on the one hand, due to the large number of differentphased current-loops (hence the a priori large number ofdegrees-of-freedom), birdcage coils are extremely complicated in termsof design and manufacturing process. It would be therefore appreciated,that the trade-off compromising in the number of degrees-of-freedomlimits the capability to tune and match the birdcage coil to the qualityfor which it has been originally designed. As opposed to the birdcagecoil, RF resonator 10 enjoys the advantage of having a small number ofcurrent-loops, hence, the procedures of matching and tuning RF resonator10 is substantially simplified over the respective procedures in priorart birdcage coils.

With respect to the slotted tube coils, in contrast to the birdcagecoils, where the theoretical number of degrees-of-freedom isimpractical, the available number of degrees-of-freedom in the slottedtube coils is too small to allow high quality imaging. Specifically,whereas slotted tube coils are equipped with tuning and matchingcircuitries, these coils fail to provide solutions to the problemsassociated with effects of field-sample interactions. A particularfeature of RF resonator 10, on the other hand, is the additionaldegree-of-freedom of balancing the RF field which is provided byelectronic circuitry 22 in addition to the tuning and matchingdegrees-of-freedom. As demonstrated in the Examples section thatfollows, the ability to balance the RF field to a symmetrical profilesubstantially minimizes inhomogeneities due to field-sampleinteractions.

The homogeneity of the RF field is achieved by a judicious selection ofboth the electrical setup connected to conductive elements 12 (e.g.,electronic circuitry 22), and the geometrical characteristics (e.g.,profile shape, dimensions, etc.) of conductive elements 12. According toa preferred embodiment of the present invention the additionalconductive elements may be added to RF resonator 10, so as to furtherimprove the magnetic field homogeneity. The effect of the additionalconductive elements on the homogeneity of the magnetic field isdemonstrated below in FIG. 12 d in the Examples section, where theadditional conductive elements are designated 13. Preferably, a phase ofthe RF current flowing through additional conductive element 13 equals aphase of the RF current flowing through conductive elements 12.

The electrical setup and the geometrical characteristics depend on thenumber of conductive elements 12. Hence, in one embodiment in which fourconductive elements are used (FIG. 8 b), the electrical setup and thegeometry are preferably such that one pair of conductive elements iselectrically and magnetically decoupled from the other pair. Thegeometrical shape of conductive elements 12 is not limited. Thus, thefirst curvature can be, for example, a curvature of cylinder, acurvature of an ellipsoid, a curvature of a hyperboloid, a curvature ofa paraboloid or a curvature of an irregular surface. According to apreferred embodiment of the present invention conductive elements 12 mayalso have a second curvature along longitudinal axis 14, which secondcurvature is also not limited and can be, for example, any of the abovecurvatures. Both the first and the second curvatures may be eitherconstant or variable curvature. For example, the first curvature mayvary with an azimuthal angle and the second curvature may vary with aninclination angle.

According to the basic laws of electromagnetism, any coil which isdesigned for generating a magnetic field induces some boundary effectsnear its edges which are manifested through inhomogeneities in themagnetic field. Therefore, the longitudinal dimension of conductiveelements 12 is preferably selected so as to minimize suchinhomogeneities to occur on the imaged object. Specifically, the edgesof RF resonator 10 are preferably designed sufficiently far from theobject to be imaged.

The separation between conductive elements 12 is preferably selectedaccording the application, so that RF resonator 10 surrounds the objectwhich is to be imaged. In one embodiment, the separation is such thatconductive elements 12 effectively surround the imaged object.

RF resonator 10 may also be coupled to RF transmitter 24, via atransmission line 26. It is often desired to minimize energy lossesbetween RF transmitter 24 and RF resonator 10. It is known that thetransferred energy between two electronic elements is maximal if theirimpedances match. Thus, preferably, the impedance of RF resonator 10 issubstantially the same as the internal resistance of RF transmitter 24.A typical value for the internal resistance of RF transmitter 24 isabout 50Ω.

With reference to FIG. 8 c, in another embodiment, the coupling betweenRF resonator 10 and RF transmitter 24 is via an RF antenna 27 (e.g.,another coil) rather than via RF transmission line 26. In thisembodiment, RF resonator 10 is wireless and all its energy istransferred from RF transmitter 24 via mutual inductance of RF antenna27 and RF resonator 10.

A well known phenomenon in the art of MRI, already discussed in theBackground section above, is that the imaged object, once loaded intothe resonator, alters many of its characteristics (e.g., the impedance,the resonance frequencies and the RF field lines), as compared to theoriginal characteristics obtained during the design of the resonator.Electronic circuitry 22 serves for tuning the resonance frequencies totheir desired values, for matching the impedance of RF resonator 10 tothe impedance of RF transmitter 24 and for balancing the RF magneticfield symmetry which is distorted by the imaged sample.

A detailed description of circuitry 22, according to preferredembodiments of the present invention, is provided herein, where thedescription of the tuning and matching precedes the description of thebalancing.

Resonance tuning and impedance matching are well known in the art, andeach independently may be done by more than one way, e.g., via tunablecapacitors, tunable inductors. In addition, the tuning and/or thematching may be done by varying mutual inductance between inductors (forexample, mutual inductance between RF resonator 10 and RF transmitter 24or mutual inductance between elements in circuitry 22 and RF transmitter24). Still in addition, the tuning and/or the matching may be done byconstructing an arrangement of transmission lines having predeterminedimpedance.

Thus, according to a preferred embodiment of the present invention,circuitry 22 comprises an arrangement of capacitors, tunable capacitors,inductors, tunable inductors and/or transmission lines, which havetypical impedance.

The capacitors used in circuitry 22 may be of any kind presently knownin the art, e.g., regular capacitors, vacuum capacitors or other highpower capacitors. Capacitors or tunable components which are expected tobe developed during the life time of this patent are not excluded.

Attention is now made to the novel concept of balancing the RF magneticfield. The spatial shape of the electromagnetic radiation fromconductive elements 12 within the volume-of-interest depends on thegeometry of the volume-of-interest and on the RF currents flowingthrough conductive elements 12 (and through additional conductiveelements 13, in the embodiments in which elements 13 are used). For agiven geometry, however, the spatial shape of the electromagneticradiation in a vacuum depends exclusively on the RF currents.

Hence, according to a preferred embodiment of the present invention,circuitry 22 is configured and designed to control the RF currentsthrough conductive elements 12 so that the generated magnetic fieldinside the volume-of-interest has a substantially symmetric profile. Forexample, in a preferred embodiment in which there are two oppositeconductive elements having identical geometry, a symmetrical profile maybe ensured, by imposing a zero electric potential on a specific point ofcircuitry 22 so that anti parallel currents in the conducting elementsare equal. Other potential configurations may also be used for providinga symmetrical magnetic RF field profile, within the volume-of-interest.

Thus, unlike, e.g., the birdcage coil, where, even when an idealhomogenous sample is used, no full compensation of the coil homogeneityis feasible once the tuning and matching capacitors are changed,electronic circuitry 22 has a particular feature of balancing the RFfield to have a substantially symmetric profile thereby to considerablyreduce sample effects. One ordinarily skilled in the art wouldappreciate that the balancing of the RF field may be achieved not onlyin cases of artificial sample but rather on real objects which are to beimaged. Moreover, for an object having a mirror symmetry (such as, butnot limited to, a human head), it is always possible to obtain asymmetric profile for the RF field, provided that the symmetry axis ofthe objects is parallel to longitudinal axis 18.

The balancing of the RF field may be done either during themanufacturing of RF resonator 10 (the so called “factory settings”), orwhile RF resonator is in medical use. Specifically, RF resonator 10preferably comprise a balancing adjuster 23 electrically communicatingwith circuitry 22 so that while in operational mode, balancing adjuster23 tunes the tunable components of circuitry 22 until the RF field has asubstantially symmetrical profile with respect to a transverse axis(i.e., perpendicular to longitudinal axis 14).

It is to be understood, however, that although RF resonator 10 has theadvantage that the resonance tuning, the impedance matching and the RFfield balancing are controlled by the same circuitry (e.g., circuitry22), other circuitries may also be employed for performing anycombination of the above adjustments.

As stated, RF resonator 10 may include any number (larger than two) ofconductive elements 12. The selected number affects the characteristicof the produced magnetic field, in particular its homogenous area andits polarization. Thus, according to a preferred embodiment of thepresent invention additional conductive elements are used to furtherimprove the magnetic field homogeneity. In this embodiment, the currentsflowing in the additional elements are related to currents 16 by apredetermined functional relation. Predetermined functions which may beused for this purpose include, but are not limited to, a linearfunction, a polynomial function, an exponential function, a rationalfunction, a power function or any combination thereof. For example, thecurrents flowing in the additional elements may be a predeterminedfraction (e.g., one half) of currents 16.

For the purpose of magnetic resonance imaging, as well as for magneticresonance analysis may be performed, many field polarization may beused, provided that the respective polarization may be mathematicallyexpanded, such that at least one component of the expansion ensures amagnetic field which rotates about the direction of the main staticmagnetic field. For example, the polarization may be circular or linear,where the linear polarization is viewed as an equal weight sum of aleft-hand circular polarization and a right-hand circular polarization.An RF resonator which generates a linearly polarized RF field is calleda linear resonator, and an RF resonator which generates a circularlypolarized RF field is called a quadrature resonator [C.-N. Chen et al.,“Quadrature Detection Coils—A Further Improvement in Sensitivity”, J.Magn. Reson., 54:324 -327, 1983].

The present invention successfully addresses the issue of polarizationof the magnetic field. Thus, according to a preferred embodiment of thepresent invention the number of conductive elements 12 is selected sothat the RF field is linearly polarized. Hence in this embodiment RFresonator 10 is a linear resonator. The linearly polarized field ispreferably produced by two conductive elements, e.g., as shown in FIG. 8a.

According to another preferred embodiment of the present invention thenumber of conductive elements 12 is selected so that the RF field issubstantially circularly polarized. Hence in this embodiment RFresonator 10 is a quadrature resonator. The circularly polarized fieldis preferably generated by four conductive elements, e.g., as shown inFIG. 8 b. In this embodiment, the four conductive elements arepreferably arranged so that perpendicularity is maintained, as furtherdetailed hereinunder.

The conductive elements may be of any known conductive material such as,but not limited to, gold, silver, cupper, copper sheets. In oneembodiment conductive elements 12 are made of a superconductingmaterial. In this embodiment RF resonator 10 further comprises means forpreserving conductive elements 12 at a sufficiently low temperature tomaintain the superconductivity. Such temperatures may range betweenabout 4.7 K and about 10 K and technologies allowing superconductivityin this contexts are described in R. S. Withers, et al., “Thin-Film HTSProbe Coils for Magnetic Resonance Imaging” SPLE Proc., Series 2156,High-T: Microwave Superconductors and Applications, 2427 January 1994,Los Angeles, Calif., 27-35, and R. D. Black, et al., “A High TemperatureSuperconducting Receiver for Nuclear Magnetic Resonance Microscopy”,Science 259: 793-95, 1993. However it is to be understood that othertechnologies, e.g., of high temperature superconductivity, which will bedeveloped during the lifetime of this patent are not excluded.

As further detailed hereinbelow, RF resonator 10 may be used, forexample, in an MRI apparatus which includes, inter alia, one or moregradient coils used. Typically, once gradient coils are switched on, arapidly growing electromagnetic field is generated, resulting ininductance of eddy currents in other surrounding conductors. Accordingto the laws of electromagnetism, these eddy currents generateelectromagnetic fields opposing the original field. Hence, according toa preferred embodiment of the present invention, conductive elements 12are designed and constructed to minimize eddy currents generatedtherein, thereby to allow the DC field generated by the gradient coilsto penetrate through conductive elements 12 once the gradient coils areswitched on. This may be done, for example, by constructing conductiveelements 12 such that each conductive elements is characterized by theso called “RF shield structure”. In other words, conductive elements 12also serve as a low-pass filter transparent to a DC field, while beingopaque to the RF field.

More specifically, conductive elements 12 may be manufactured as twoconductive foils (e.g., copper foils, or superconducting foil) having adielectric material therebetween, thereby forming a double sidedconductive surface. The formation of eddy currents may then be preventedby etching narrow gaps on each side of the double sided conductivesurface such that no two gaps overlap. Thus, when the narrow gaps arefilled with a dielectric material having the appropriate properties(e.g., dielectric constant and thickness), the gaps, while beingtransparent to the DC gradient field, are still opaque to the RFcurrents. It is to be understood, however, that unlike the birdcagecoil, where currents flowing through the legs of the birdcage coil arewith different phases, the RF currents flowing in each one of conductiveelements 12 are all in one phase.

The present invention successfully provides a method of designing an RFresonator for magnetic resonance analysis, for example RF resonator 10.The method comprises the following method steps which are illustrated inthe flowchart of FIG. 9.

Hence, in a first step, designated in FIG. 9 by Block 32, at least twocomputationally defined surfaces are selected for engaging at least twoconductive elements (e.g., conductive elements 12). The surfaces have afirst curvature along a direction perpendicular to a longitudinal axis,and may have a second curvature along the longitudinal axis. The numberof surfaces (and therefore also the number of conductive elements) ispreferably selected according to the desired polarization of the RFfield, as further detailed hereinabove and exemplified in the Examplessection that follows. The number of surfaces also dictates the number ofeigenvectors (current-modes) which appear in the solution of thecorresponding eigenvalue equation for the currents flowing through theconductive elements. Specifically, the number of surfaces equals thenumber of eigenvectors, thereby to the number of eigenvalues, which, asalready explained in the Background section hereinabove, are related tothe resonance frequencies of the RF resonator.

Unlike birdcage coil, where the number of legs (therefore also thenumber of undesired resonance frequencies which are to be eliminated) islarge, in the RF resonator of the present invention, the number ofconductive element is preferably selected so as to minimize the overallnumber of current-modes. Thus, for small number of conductive elements,there are a small number of current-modes corresponding to a smallnumber of resonance frequencies. Being sufficiently spaced apart in thefrequency space, the desired mode may be easily selected, leaving theundesired modes to be substantially inactive.

In a second step of the method, designated by Block 34, a magnetic fieldis calculated within the surfaces. Unlike the design of prior art coils(e.g., birdcage coils), the design of the magnetic field is completelydecoupled from the design of the resonance characteristics of theresonator. Thus, in a third step, designated by Block 36 in FIG. 9, thefirst and the second steps are iteratively repeated so as to provideoptimize geometry which corresponds to a substantially homogenousmagnetic field. Specifically, in each iterative step, any specificcombination of curvature, length, separation and number of conductiveelements may be tested to optimize the homogeneity of the magneticfield.

In a fourth step, designated by Block 38, means capable of providingpredetermined resonance characteristics of the RF resonator, areselected based on the optimize geometry and the magnetic field which arealready known from the third step. Preferably, frequency shifts whichmay occur due to the future existence of an imaged object within the RFcoils are considered while the fourth step is executed. Stillpreferably, impedance differences which may be present between the RFresonator and the RF transmitter are considered in the forth step. Thus,the means which are selected preferably comprise an arrangement oftunable components which can be used for impedance matching. Furtherpreferably, the profile of the RF field with respect to a transverseaxis is considered in the fourth step. In other words, the means whichare selected preferably designed to facilitate the above-mentionedprocess of balancing the RF field lines, so as to have a substantiallysymmetrical profile with respect to a transverse axis

It is to be understood that the execution of the fourth step is farsimpler than an equivalent step in the design of, e.g., a birdcage coil,due to the relatively small number of current-modes. Specifically, asfurther demonstrated in the Examples section that follows, it issufficient to select a small number of tunable components in the designof the RF resonator, so that once the RF resonator is reduced topractice, these tunable components are used for tuning, matching andbalancing the RF resonator. Although, the tuning, matching and balancingare, in principle, repeated iteratively, one would appreciate that asthe number of tunable components is minimized the simplicity of findingan optimal solution is maximized.

The second step of the method may be executed in more than one way. Inone embodiment, the second step is executed straightforwardly by solvingthe set of Maxwell's equations within the geometry selected in the firststep, thereby to obtaining electromagnetic field lines.

In another embodiment, the second step is executed by a method known asthe finite element method. The finite element method is described, forexample, in a book by Jianming Jin, entitled “Electromagnetic Analysisand Design in Magnetic Resonance Imaging”, published by CRC press LLC,the contents of which are hereby incorporated by reference. Thisembodiment is preferably used in cases in which the size of the RFresonator is smaller than the desired resonant wavelength. The basicprinciple of this method is to divide the space into a plurality offinite elements (e.g., rectangular elements, triangular elements and thelike) and to use these elements for solving the governing equations.

In an additional embodiment, the second step is executed by a methodknown as the moments analysis method, which is also described, e.g., inthe book of Jianming Jin (ibid). This embodiment is preferably used whenthe size of the RF resonator is comparable to the desired resonantwavelength. The basic principle of the moments analysis method is toformulate and solve an integral equation representing the fields,produced by currents flowing in the RF resonator, using a free-spaceGreen's function.

The RF resonator, designed according to the above method steps, may beused, e.g., in an MRI apparatus combining the RF resonator, a device forproviding a static magnetic field and maybe also one or more gradientcoils. It is known that when the RF field electromagnetically interactswith the static magnetic field device and/or with the gradient coil(s)energy losses may occur, degrading the performances of the RF resonator.In addition, such electromagnetic interaction may shift the resonancefrequency or even add more current-modes which are characterized byresonance frequencies which are close to the desired mode frequency,hence reduces the ability of the RF resonator to operate in a single,desired, resonance frequency.

Thus, according to a preferred embodiment of the present invention, themethod further comprises a step of designing an RF shield for minimizingelectromagnetic interactions between the RF resonator and the staticmagnetic field device and/or between the RF resonator and the gradientcoil(s). This step is preferably executed prior to the second step andit is represented by Block 44 in FIG. 9. RF shields are known in the artand their design is straightforward once the RF resonator configurationis known. One method of designing an RF shield is known as the method ofimages, and is found, e.g., in an article by Jin J. M. et al., entitled“A Simple Method to Incorporate the Effects of an RF Shield into RFResonator Analysis for MRI Applications”, published in IEEE Trans.Biomed. Eng., 42:840-843, the contents of which are hereby incorporatedby reference. Other methods are found in Ong K C, et al.,“Radiofrequency shielding of surface coils at 4.0T” Magn. Reson. Imag.5:233-237, 1987; and in Alecci M, “Characterization and reduction ofgradient-induced eddy currents in the RF shield of a TEM resonator”,Magn Res in Medicine 48:404-407, 2002.

In the method of images one assumes that the field produced by currentsinduced on the RF shield is a mirror image of the field produced by theoriginal currents. Thus, given the original currents, one calculates thetotal RF field produced both by the original and by the inducedcurrents.

According to a preferred embodiment of the present invention the methodfurther comprises a step of designing at least one end-cap to bepositioned adjacent to at least one end of the RF resonator forminimizing magnetic field inhomogeneities along the longitudinal axis.This step is preferably executed prior to the second step and it isdesignated by Block 46 in FIG. 9. End-caps are known in the art ofmagnetic resonance design and are used as “electromagnetic mirrors”which effectively increase the longitudinal dimension of the resonator.Thus, the end-cap minimizes boundary effects, hence reduces the fieldinhomogeneity, in particular near the edge where the end-cap ispositioned.

According to an additional aspect of the present invention, there isprovided an RF resonator for magnetic resonance analysis generallyreferred to herein as RF resonator 100. Hence, RF resonator 100comprises at least two conductive elements 12, and at least oneadditional conductive element 13. The principles and operations of RFresonator 10 are similar to the principles and operations of RFresonator 10, as further detailed hereinabove. Conductive elements 12and additional conductive elements 13 of RF resonator 100 are bestillustrated in FIG. 12 d of the Examples section hereinbelow.

Reference is now made to FIG. 10, which is a block diagram illustratingan MRI apparatus, generally referred to as apparatus 50, according to anadditional aspect of the present invention.

Hence, apparatus 50 comprises a device 51 for providing a staticmagnetic field. Device 51 may be, for example, a permanent magnet, anelectromagnet, a superconductivity-based device and the like. It isexpected that during the life of this patent many relevant magnets andelectromagnets will be developed and the scope of the phrase “device forproviding a static magnetic field” is intended to embrace all such newtechnologies a priori. For example, device 51 may comprises at least oneshim coil for further corrections of the homogeneity of the staticfield.

Apparatus 50 further comprises a processing unit 52, an RF transmitter54 and an RF resonator 56. The construction and operation of RFresonator 56 are similar to the construction and operation of RFresonator 10 or RF resonator 100 as further detailed hereinabove. RFresonator 56 may also have supplementary features with respect to RFresonator 10, for example, RF resonator 56 may be a multi frequency RFresonator designed for obtaining signals from two or more nuclei, eachhaving a different gyromagnetic ratio (e.g., 1H and 31P). Additionally,RF resonator 56 may include conductive elements having a predeterminedcapacitance distribution, that reduce the electric field created by RFresonator 56, for minimizing effects of the imaged object on themagnetic field and for minimizing corona discharge from conductiveelements 12.

According to a preferred embodiment of the present invention apparatus50 may further comprise any number of gradient coils 58 for providing apredetermined gradient of static magnetic field. Any gradient coil knownin the art of MRI may be used. As stated, electromagnetic interactionsbetween the RF field and device 51 and/or between the RF field and thegradient coil(s) 58 result in undesired energy losses. Thus, apparatus50 further comprises an RF shield 60, configured and designed forminimizing those electromagnetic interactions, as further detailedhereinabove. Apparatus 50 may also comprise one or more end-caps 62,positioned adjacent to one or more ends of RF resonator 56 forminimizing magnetic field inhomogeneities.

According to a preferred embodiment of the present invention apparatus50 further comprises decoupling means for electrically decoupling RFresonator 54 from one ore more of gradient coils 58. This may be done,for example, by an enveloping external device having high valuecapacitors located thereon. Such a device is known as a DC blockcapacitors.

According to a preferred embodiment of the present invention apparatus50 may include more than one RF resonator 56. For example, RF resonator56 and one or more additional resonators may be arranged to form an RFresonator array, such as, but not limited to, a phased array.Preferably, all the RF resonators are electromagnetically decoupled fromeach other to avoid the undesired effect of interactions between thegenerated RF fields.

Reference is now made to FIG. 11, which is a flowchart of a method formagnetic resonance analysis of an object. The method comprises thefollowing method steps in which in a first step, represented by Block72, a static magnetic field is applied on the subject in a direction ofa longitudinal axis. In a second step, represented by Block 74,substantially homogenous RF magnetic field is applied on the subject,using, e.g., RF resonator 10, 100 or 56, in a direction perpendicular tothe longitudinal axis. In a third step, represented by Block 76, nuclearmagnetic resonance parameters are acquired from the object, so as toanalyze the object.

The MRI apparatus, the method of magnetic resonance analysis and anyapparatus, device and/or system which employs the RF resonator describedabove may be employed on many objects which are to be imaged and/oranalyzed. These include, but are not limited to, an animal (e.g., ahuman being), an organ of an animal, a tissue, a swollen elastomer and afood material. In addition, the object may be at least one type ofmolecules present (e.g., dissolved, dispersed or emulsed) in thesolvent.

Additional objects, advantages, and novel features of the presentinvention will become apparent to one ordinarily skilled in the art uponexamination of the following examples, which are not intended to belimiting. Additionally, each of the various embodiments and aspects ofthe present invention as delineated hereinabove and as claimed in theclaims section below finds experimental support in the followingexamples.

EXAMPLES

Reference is now made to the following examples, which, together withthe above descriptions, illustrate the invention in a non limitingfashion.

Example 1 A Linear RF Resonator

A prototype of a linear resonator having two conductive elements wasdesigned and built for imaging a rat head in an 8.46 T static magneticfield generated by a AVANCE 360 WB spectrometer, purchased from BrukerMedizintechnik GmbH, D-76275 Ettlingen, Germany.

Description of the Prototype

The ratio between the resonator size and the resonance wavelength wasselected to be about 0.02, hence, any variation of the RF phase wasnegligibly small (the so called “near-field approximation”). Themathematical reason for the smallness of variation is that the variationis proportional to the sine of the distance from the conductive element,hence, for distances which are sufficiently smaller than the wavelength,the sine (and the variation) approaches zero. Specifically, the distancefrom the conductive surfaces to the center of the imaged sample was 1.5cm and the resonant wavelength was 83.2 cm. For such configuration thechange in the wave amplitude over the sample is less than one percent. Anon conductive gap of 2.5 mm between the conductive elements and thevolume-of-interest was designed. The dimensions of the gap were selectedfor avoiding strong field effects near the conductive elements on theone hand, and for maintaining a sufficiently effective the pulse lengthon the other hand. The RF resonator was designed so as to be envelopedby a cylindrical shell of a gradient unit, 53 mm in diameter.

RF Field Simulation

The Laplace equation governing the static electric component of thefield inside the RF resonator volume was solved by a finite differencemethod, utilizing Simlon 6.0 software.

For simplicity, infinitely long legs were assumed, thereby neglectingboundary effects the RF resonator has antiparallel mirror symmetry.Thus, in accordance with the uniqueness theorem of the Laplace equation,one half of the RF resonator was simulated and the other half wasmirrored by a straight plane of zero field intensity.

The cylindrical container of the RF resonator served as an effective RFshield, hence simulated by a zero-potential half-circle.

Reference is now made to FIGS. 12 a-c, illustrating the calculatedequipotential lines of the field, for various profiles of conductiveelements 12. One ordinarily skilled in the art would appreciate that theequipotential lines also represent the pattern of the magnetic field forthis simulation. FIGS. 12 a and 12 b show profiles which generate inhomogenous field. FIG. 12 c shows an optimal profile generating acentral area of equally spaced and substantially parallel equipotentiallines corresponding to a homogenous field. In this configuration aplanar angle of 100° was measured from the center of the RF resonator tothe edges of each conductive element.

FIGS. 12 d-e show simulations results in which additional conductiveelements 13, carrying a current which is one half of the current flowingin conductive elements 12, are used in addition to conductive elements12. FIG. 12 d shows equipotential lines and FIG. 12 e shows field lineswhere each field line represents an intensity step of 9%. It can be seenthat additional conductive elements 13 serve for controlling the fieldhomogeneity.

FIG. 12 f shows results of electric field simulations of the prototyperesonator. The simulations were executed using Matlab™ 6 softwarepackage, using a 120×120=14400 elements matrix. FIG. 12 e is a contourplot of the electric field gradient, which represents the RF magneticfield.

A circular area-of-interest located in the central area of the prototyperesonator was defined, which area-of-interest included 1130 out of the14400 elements, and is equivalent to a circle 22 mm in diameter. Thesimulations were directed at calculating statistical characteristics ofthe area-of-interest. As the simulations are inherently symmetrical, itis sufficient to simulate one half of the transverse plane. As furtherdemonstrated hereinafter, the same area-of-interest was defined forexperimental measurements, where the resolution is about 44% higher(128×256 pixels).

The uniformity of the RF magnetic field was defined as: $\begin{matrix}{{{Uniformity} = {1 - \frac{\sigma}{\overset{\_}{I}}}},} & \left( {{EQ}.\quad 1} \right)\end{matrix}$

where σ is the intensity standard-deviation and I is the averageintensity in the analyzed area. The uniformity was found to be 96% forthe above described area of interest.

AC Simulations

The resonance characteristics were designed using the lumped elementsmethod, employing Micro-Cap 6 software for the AC analysis. Possibledeviation between the simulation process and the physical RF resonatorwere considered a-priori by introducing three tunable capacitors intothe circuit.

FIG. 13 illustrates the simulated circuit. L1 and L2 are inductorsrepresenting conductive elements 12. C1, . . . , C5 are capacitors, R1,R2, R3 and R9 are parasite resistances V1 is an RF transmitter and K1 isthe mutual inductance between L1, L2 and was set to be 0.3. The maincurrent loop defined by elements L1, C1, C2 and L2 envelopes the sampleto be imaged. Capacitors C3 and C4 were used for balancing between theleft portion and the right portion of the circuit, while capacitor C5was used for matching the circuit to RF transmitter V1. In thesimulation, the current flowing through capacitors C3 and C4 wasinductively decoupled from the main current-loop. In other words, themagnetic field that is generated by the current through capacitors C3and C4 was not interacting with the sample. Resistor R9 was introducedfor representing the sample's resistance. The simulations results areprovided hereinbelow with reference to points-of-interest, designated inFIG. 13 by 1, 2 and 6.

FIG. 14 show the simulated potential difference between points 1 and 2,and the potential at point 6. The RF resonator has only two currentmodes, referred to herein as a common-mode and an anti-mode,corresponding to resonance frequencies of 310 MHz and 360 Mhz,respectively. The desired mode frequency was selected to be the 360 Mhzmode. Both resonance frequencies are shown as voltage peaks in FIG. 14.The frequency separation between the two modes is governed by therelative values of capacitors C1, C2, C3 and C4, and by the mutualinductance between conductive elements 12.

As further demonstrated below, the physical circuit can be matched,balanced and tuned to the desired mode, using three tunable capacitors(C1, C3 and C4) and one fixed-value capacitor (C2), configured so as toeliminate the voltage drop on resistor R9, i.e., imposing a zero voltageat a point designated “A” in FIG. 13. In practice, while imaging asample, the zero voltage at “A” minimizes transient effects on thecircuit realized by muscles movements (breath, blood pulsation, etc.) ofthe sample, and ensures that the two halves of the circuit are balanced.

Prototype Buildup

Materials and Methods

Reference is now made to FIG. 15, showing the experimental setup. Theprototype included two conductive elements 12, capacitors 82, an uppermetal washer plate 84, a lower metal washer plate 94, insulators 86, anexternal cylinder 88 and an internal cylinder 90. The terminalsconductive elements 12 are designated 92.

The experimental setup was build from brass sheet metal processed bymachining. The external container was 53 mm in diameter, conductiveelements 12 were 1 mm thick and 25 mm height and their edges wererounded. Conductive elements 12 situated along an arc, 30 mm indiameter.

All circuitry connections and capacitors were located above conductiveelements 12. Metal washer plate 84 was used to screen all capacitors andconnectors from the imaging volume, so as to minimize the fieldcontribution from sources other than the conductive elements 12, and inparticular the abovementioned current-loops of C3, and C4. The tunablecapacitors were selected of low self resistance compared to the sample'sresistive load. Thus, only small amount of power was dissipated throughthe C3 and C4 current-loops.

Both conductive elements 12 were connected by welding to a commonground. Terminals 92 protrude into the capacitors level through slots inupper metal washer plate 84. Conductive elements 12 were electroplatedwith 10 μM of silver, in order to increase conductivity within the skindepth range. The Silver layer was electroplated with a protective 1 μmlayer of Gold. Most other parts of the probe were made of TEFLON™, suchas the internal pipe that prevents direct contact between the sample andthe conductive elements 12. Ertalon 4.6 nuts were used to fix thetunable capacitors in place. The circuit uses 3 tunable capacitorsNMAP40HV purchased from Voltronics, range from 1.5 to 40 μF (C1, C3 andC4 of FIG. 13) and one chip capacitor of 2.2 pF purchased from ATM (C2of FIG. 13). Once assembled, the circuit was tuned, matched and balancedat the resonant frequency of 360.13 MHz. Balance was achieved byimposing a zero voltage at “A”.

The resonant spectrum of the experimental setup was checked by means ofa spectrum analyzer and found to match the predictions of the ACsimulations (see FIG. 14). Identification of the undesired common modewas performed by touching “A” with a metal instrument, thus affectingonly the undesired mode while having no effect on the desired mode. Whenthe desired mode was set to 360.13 MHz, the undesired common mode'sfrequency was 236.5 MHz, showing higher mutual inductance betweenconductive elements 12 than the original simulation estimates.

For the purpose of obtaining a full mapping of the RF magnetic fieldintensity, a sufficiently large phantom was selected. The phantom was acontainer filled with 0.075 M saline. The phantom length was more than 5cm, which is much longer than the effective imaging volume of the RFresonator.

An unloaded of Q≈70 was estimated for the experimental setup using asmall loop antenna. The extent of the sample effect on the circuit wasmeasured by comparing the resonant frequency of the circuit with andwithout the phantom. As the phantom occupies the entire RF resonatorvoid, it bears much greater resistance and capacitance than the samplewhich is to be imaged. The difference between the two frequencies was0.8 MHz, which is very small once considering the circuit Q value.

Imaging Tests

A spin echo pulse sequence was chosen to for the excitation, so as tomaximize the dependence of the signal intensity on the RF magneticfield. One ordinarily skilled in the art would appreciate that, asopposed, for example to a 90° pulse, such pulse substantially overestimates this dependence. Thus, the results presented herein are to beconsidered as bounding the quality of the prototype from below.

The uniformity of the RF magnetic field was defined as in Equation 1hereinabove.

FIGS. 16 a-b show the RF magnetic field profile trough an axial slice,and the one-dimensional cross sections along orthogonal directions. Theyellow circle represents an area-of-interest, 22 mm in diameter, towhich statistical characteristics were calculated. As opposed to thesimulations of the area-of-interest, the image is more vulnerable tonon-symmetrical effects. FIG. 16 a shows plots of the measured magneticfield along two one-dimensional profiles, which are indicated as twoperpendicular lines with respective colors within the area-of-interest.The calculated uniformity, in accordance to Equation 1, for thearea-of-interest was 95%, in agreement with the uniformity calculated inthe simulations. FIG. 16 b shows the calculated statisticalcharacteristics and the histogram for the area-of-interest. Generally,the RF magnetic field pattern is highly homogenous in the central areasof the coil.

FIGS. 17 a-b show the RF magnetic field profile trough a coronal slice.A yellow rectangle, 14×21 mm, represents an area-of-interest, to whichstatistical characteristics were calculated. FIG. 17 a shows plots ofthe measured magnetic field along two one-dimensional profiles, whichare indicated as two perpendicular lines with respective colors withinthe area-of-interest. FIG. 17 b shows the calculated statisticalcharacteristics and the histogram for the rectangular area-of-interest.The profile along the transverse direction is substantially homogenousin the central imaging area. The profile along the longitudinaldirection shows a homogenous area in the central area of the RFresonator, with a remnant region of boundary effects near the edges.

Although boundary effects were detected, the calculated uniformitywithin the area-of-interest was 96%. The reader is advised to comparethe observed boundary effects with other RF coils, e.g., the 16 legsbirdcage shown in FIG. 4.20 (page 172) of the book of Jianming J.(ibid). The boundary effects in the birdcage are more pronounced, eventhough, in principle, the dimensions of the birdcage favor much lessboundary effects due to the lower diameter-to-height ratio, as comparedto the present prototype.

FIGS. 18 a-b show the RF magnetic field profile trough a sagittal slice,within the same area-of-interest as in FIGS. 17 a-b. FIG. 17 a showsplots of the measured magnetic field along two one-dimensional profiles,which are indicated as two perpendicular lines with respective colorswithin the area-of-interest. FIG. 17 b shows the calculated statisticalcharacteristics and the histogram for the rectangular area-of-interest.Similarly to the coronal slice, the profile along the transversedirection is substantially homogenous in the central imaging area, whilea remnant region of boundary effects is observed along the longitudinaldirection near the edges. The uniformity for the above rectangle ofinterest was 90%. The difference between the sagittal (FIGS. 18 a-b) andthe coronal (FIGS. 17 a-b) slices uniformity is explained by the mirrorsymmetry of the prototype.

FIGS. 19 a-d show axial spin echo slices in a rat head, using an 8.46 Twide bore magnet and utilizing fat suppression pulses. Each slicethickness was 1 mm. The quality of the image is vivid.

Example 2 A Quadrature RF Resonator

Following is a description of a quadrature RF resonator designed as twodecoupled linear RF resonators.

Reference is now made to FIG. 20 illustrating a top view of thequadrature RF resonator of this example. The quadrature resonator ofthis example includes four conductive elements arranged as two pairs,designated 12 and 12 a. Pair 12 and pair 12 a are positioned so that therespective symmetry axes are perpendicular to each other. Antiparallelcurrents 16 are driven through pair 12 and pair 12 a, where, accordingto the common convention, current which is outgoing downwards (into theplane of FIG. 20) is designated by the cross symbol and current which isincoming upwards is designated by the dot symbol. Currents 16 are drivenby a split phase RF source, so that the voltage on pair 12 is shifted by90° with respect to the voltage on pair 12 a.

Currents 16 generate magnetic field in and outside the volume surroundedby the conductive elements. The characteristics of the magnetic fieldgenerated by pair 12 on the surface of pair 12 a may be betterunderstood by considering two points, designated G and F, located onopposite sides of one conductive element of pair 12 a, which point aresymmetrical with respect to the pair 12. The magnetic field at G and Fis identical in amplitude and opposite in sign, due to the oppositedirection of the currents near each point. One ordinarily skilled in theart would appreciate that similar consideration apply for each twosymmetrical located pair of points along one conductive element of pair12 a. Thus, the magnetic field on the surface of each conductive elementof pair 12 a is antisymmetric with respect to a symmetry axis of thesurface.

Reference is now made to FIG. 21 which shows the electrical currentsinduced onto the surfaces of pair 12 a, due to changes in magnetic fluxtherethrough. At each point of points F and G, the induced current isopposite in direction to that of the conductive element of pair 12 whichis close to it. Therefore, the effect of pair 12 on pair 12 a is theinduction of current loops by on the surface pair 12 a.

However, although current loops do exist on pair 12 a, the total netcurrent induce by pair 12 on pair 12 a, is zero, thereby ensuring thatpair 12 a is decoupled from pair 12. A skilled artisan would appreciatethat such decoupling may be achieved, for different geometries of thesurfaces, provided that each conductive element is symmetrical, and thattwo pairs are perpendicularly positioned (e.g., as in FIG. 20).

The driven currents flowing through pair 12 a generate an inducedmagnetic field directed opposite to the direction of the magnetic fieldgenerated by pair 12. Hence, even though no electric/magneticconjugation exists between the pairs, pair 12 a changes the fieldpattern generated by pair 12. Specifically the use of pair 12 a inaddition to pair 12, under a phase shift condition which is supplied bythe split phase RF source, ensures that the RF magnetic field issubstantially circularly polarized.

The magnetic field pattern of the quadrature RF resonator of the presentexample was simulated utilizing the same technique that was used for thedetermination of the linear RF resonator of Example 1. In thesimulations, the contribution of pair 12 a to the magnetic field wascalculated by representing the conductive elements of pair 12 a assurfaces of zero magnetic field. The justification for thisrepresentation is related to the observation that due to its shortwavelength, the RF field cannot percolates through a conductive elementwhose width is larger than many skin depths.

As stated the simulation is conducted using the near-fieldapproximation, hence, pair 12 a is represented by zero-potentialsurfaces, while the pair 12, is represented by a finite potentialsurface. The voltage configuration of the quadrature RF resonator isshown in FIG. 22.

Reference is now made to FIGS. 23 a-b, showing the simulation results.FIG. 23 a shows the calculated electric potential and FIG. 23 b showsthe calculated electric field. As can be seen, a considerable homogenousarea with a substantially zero gradient was observed in the centralregion of the quadrature RF resonator.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention, which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable subcombination.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims. All publications, patents and patentapplications mentioned in this specification are herein incorporated intheir entirety by reference into the specification, to the same extentas if each individual publication, patent or patent application wasspecifically and individually indicated to be incorporated herein byreference. In addition, citation or identification of any reference inthis application shall not be construed as an admission that suchreference is available as prior art to the present invention.

1-312. (canceled)
 313. A radiofrequency (RF) resonator for magneticresonance analysis, the RF resonator comprising: (a) at least twoconductive elements, each having a first curvature along a directionperpendicular to a longitudinal axis, said at least two conductiveelements being spaced along said longitudinal axis, so that when an RFcurrent flows within said at least two conductive elements in adirection of said longitudinal axis, a substantially homogenous RFmagnetic field, directed perpendicular to said longitudinal axis, isproduced in a volume defined between said at least two conductiveelements; and (b) at least one additional conductive element,electrically communicating with said at least two conductive elements ina manner such that a phase of an RF current flowing through said atleast one additional conductive element equals a phase of at least oneof said RF currents flowing through said at least two conductiveelements.
 314. The RF resonator of claim 313, further comprising anelectronic circuitry designed and configured for providing predeterminedresonance characteristics of the RF resonator, for matching an impedanceof the RF resonator to an impedance of an RF transmitter electricallycommunicating with said electronic circuitry, and for balancing said RFmagnetic field to have a substantially symmetrical profile with respectto a transverse axis being perpendicular to said longitudinal axis. 315.An apparatus for magnetic resonance analysis, the apparatus comprising:(a) a device for providing a static magnetic field; (b) a processingunit; and (c) a radiofrequency (RF) resonator coupled to an RFtransmitter, said RF resonator comprising: at least two conductiveelements, each having a first curvature along a direction perpendicularto a longitudinal axis, said at least two conductive elements beingspaced along said longitudinal axis, so that when an RF current flowswithin said at least two conductive elements in a direction of saidlongitudinal axis, a substantially homogenous RF magnetic field,directed perpendicular to said longitudinal axis, is produced in avolume defined between said at least two conductive elements; and atleast one additional conductive element, electrically communicating withsaid at least two conductive elements in a manner such that a phase ofan RF current flowing through said at least one additional conductiveelement equals a phase of at least one of said RF currents flowingthrough said at least two conductive elements.
 316. The apparatus ofclaim 315, wherein said RF resonator further comprising an electroniccircuitry designed and configured for providing predetermined resonancecharacteristics of said RF resonator, for matching an impedance of saidRF resonator to an impedance of said RF transmitter, and for balancingsaid RF magnetic field to have a substantially symmetrical profile withrespect to a transverse axis being perpendicular to said longitudinalaxis.
 317. The apparatus of claim 316, wherein said RF resonator furthercomprising a balancing adjuster electrically communicating with saidelectronic circuitry, said balancing adjuster is constructed anddesigned for controlling said electronic circuitry while said RFresonator is in medical use.
 318. The apparatus of claim 315, whereinsaid device for providing said static magnetic field comprises at leastone shim coil.
 319. The apparatus of claim 315, further comprising atleast one gradient coil.
 320. The apparatus of claim 315, furthercomprising at least one end-cap positioned adjacent to at least one endof said RF resonator, said at least one end-cap constructed and designedfor minimizing magnetic field inhomogeneities along said longitudinalaxis.
 321. The apparatus of claim 316, wherein said electronic circuitrycomprises means for varying mutual capacitance.
 322. The apparatus ofclaim 316, wherein said electronic circuitry comprises means for varyingmutual inductance.
 323. The apparatus of claim 322, wherein said mutualinductance is defined between said RF resonator and said RF transmitter.324. The apparatus of claim 322, wherein said mutual inductance isdefined between said electronic circuitry and said RF transmitter. 325.The apparatus of claim 315, wherein a longitudinal dimension of said atleast two conductive elements is selected so as to minimize magneticfield inhomogeneities along said longitudinal axis.
 326. The apparatusof claim 315, wherein a separation between said at least two conductiveelements is selected so as to surround an object to be imaged.
 327. Theapparatus of claim 326, wherein said object is a mammal, an organ of amammal, a tissue, a swollen elastomer, a food material, a liquid, and atleast one type of molecules present in the solvent.
 328. The apparatusof claim 315, wherein at least one of said at least two conductiveelements further has a second curvature along a direction parallel tosaid longitudinal axis.
 329. The apparatus of claim 315, wherein anumber of said at least two conductive elements is selected so that saidsubstantially homogenous RF magnetic field is linearly polarized. 330.The apparatus of claim 315, wherein a number of said at least twoconductive elements is selected so that said substantially homogenous RFmagnetic field is substantially circularly polarized.
 331. The apparatusof claim 315, wherein said at least two conductive elements are twoconductive elements.
 332. The apparatus of claim 315, wherein said atleast two conductive elements are four conductive elements.
 333. Theapparatus of claim 332, wherein a first pair of said four conductiveelements is electrically decoupled from a second pair of said fourconductive elements.
 334. The apparatus of claim 332, wherein a firstpair of said four conductive elements is magnetically decoupled from asecond pair of said four conductive elements.
 335. The apparatus ofclaim 332, wherein a first pair of said four conductive elements iselectrically decoupled from a second pair of said four conductiveelements.
 336. The apparatus of claim 332, wherein a first pair of saidfour conductive elements is electromagnetically decoupled from a secondpair of said four conductive elements.
 337. The apparatus of claim 332,wherein a first pair and a second pair of said four conductive elementsare positioned so that a transverse axis of said first pair issubstantially perpendicular to a transverse axis of said second pair.338. The apparatus of claim 315, wherein said at least two conductiveelements are made of a superconducting material.
 339. The apparatus ofclaim 338, further comprising means for preserving said at least twoconductive elements at a sufficiently low temperature.
 340. Theapparatus of claim 338, further comprising at least one additional RFresonator arranged with said RF resonator to form an RF resonator array.341. The apparatus of claim 340, further comprising decoupling means fordecoupling said RF resonator from said at least one additional RFresonator.
 342. The apparatus of claim 319, further comprisingdecoupling means for decoupling said RF resonator from said at least onegradient coil.
 343. The apparatus of claim 340, wherein said array is aphased array.
 344. The apparatus of claim 338, wherein said RF resonatoris a multi frequency RF resonator.
 345. The apparatus of claim 338,wherein each of said at least two conductive elements has apredetermined capacitance distribution for minimizing effects of anobject to be imaged on said magnetic field and for minimizing coronadischarge from said at least two conductive elements.
 346. The apparatusof claim 315, wherein said at least two conductive elements are designedand constructed to minimize eddy currents generated therein.
 347. Theapparatus of claim 315, wherein said at least two conductive elementsare characterized by an RF shield structure for substantially blockingRF radiation while transmitting low frequency radiation.